Method for modifying surface of metal bipolar plate and bipolar plate for fuel cell

ABSTRACT

A bipolar plate for a fuel cell is provided, which includes: a metal substrate having a flow field structure; a conducting adhesion layer formed on the metal substrate and having a polymeric adhesive and a plurality of conductive particles; and a pure graphite layer formed on the conducting adhesion layer and structurally corresponding to the flow field structure of the metal substrate. The graphite layer including expanded graphite powder is adhered to the metal substrate via the conducting adhesion layer, and a portion of the expanded graphite powder is embedded into the conducting adhesion layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.13/964,636, filed on Aug. 12, 2013, claiming priority to TaiwanesePatent Application No. 101129682, filed on Aug. 16, 2012, the disclosureof which is hereby incorporated by reference herein.

TECHNICAL FIELD

This disclosure relates to methods for modifying the surface of a metalbipolar plate and bipolar plates for fuel cells, and particularly, to amethod for modifying the surface of a metal plate by using graphite anda bipolar plate having a graphite layer.

BACKGROUND

Fuel cells are highly efficient, safe in operations, and low inpollution. Therefore, they are applied in a variety of fields, such aselectric power, industry, transportation, aeronautics, military, and thelike. A fuel cell is a power generating device, which can continuouslyand directly converting chemical energy into electrical energy. When afuel cell is in operation, a fuel gas (such as hydrogen gas) and acombustion promoter (such as oxygen gas) are delivered to the anode andcathode of the fuel cell, respectively. Oxidation and reduction thentake place to convert chemical energy into electrical energy.

The structure of a conventional fuel cell unit is substantiallyconsisted of an anode plate, a cathode plate, and a solid electrolytefilm interposed between the anode plate and the cathode plate, and isreferred to as a battery cell. However, in practical uses, multiplebattery cells may be connected in series, so as to achieve a greateroutput voltage. Adjacent fuel cell units have a common electrode plate,which serves as the anode and cathode of the two adjacent fuel cellunits, respectively. Thus, the electrode plate is referred to as abipolar plate.

Currently, in the structure of a bipolar plate for a fuel cell, apolymeric material is overlaid on a stainless steel substrate byspray-coating, and the polymeric material is bonded to the substrate asa result of pyrolysis. At least 90% of conductive graphite is added tothe polymeric material to block corrosion and oxidation caused by theexternal environment to the stainless steel substrate, and to conferconductivity to the stainless steel substrate. However, the conductiveprotective film is formed by spray-coating, whereby the solvent in thepolymeric material in the film evaporates during pyrolysis, such thatnumerous small air bubbles are generated in the conductive protectivefilm finally formed. This causes the conductive protective film to havepoor density. Further, the small air bubbles serve as channels for theinfiltration of the acidic solution of a fuel cell into the bipolarplate. Moreover, film formation on the overlaid layer formed by acoating process is ineffective at a specific angle on the flow fieldstructure, because the flow field structure at the surface of the metalsubstrate have a complex structure along the horizontal and verticaldirections. Hence, multiple layers of polymeric materials must first becoated on the surface of the stainless steel substrate, and thenconducting a number of pyrolytic processes, in order to avoid theoccurrence of the aforesaid problems. Nevertheless, this makes theprocessing procedures for the bipolar plate be too complicated, and alsomakes the production cost be too expensive.

Further, a gas separator, which uses a tin paste to bond a graphitelayer to a stainless steel base, has been developed. Specifically,heating and pressurizing are utilized to bond the graphite layer to thestainless steel substrate via an interposed tin layer. However, thetin-containing gas separator would poison a fuel cell and the graphitelayer has hetero-junction with the tin layer, such that delaminationoccurs among the layers due to poor strengths. Moreover, acid solutionmay permeate the graphite layer to corrosion the tin layer and poisonMEA to make PEM fuel cell break down.

In addition, a bipolar plate comprised of a resin material has beendeveloped which involves the preparation of a plurality of molded sheetscontaining carbon materials, and then press-fitting of the molded sheetto obtain the bipolar plate. However, the problem of poor air tightnessof the bipolar plate results from the control of the composite carbonboard to extreme thinness.

SUMMARY

The present disclosure provides a method for modifying a surface of ametal bipolar plate, including the steps of providing a metal substratehaving a conducting adhesion layer on the surface thereof, the metalsubstrate having a flow field structure at the surface thereof; applyingexpanded graphite powder onto the conducting adhesion layer; andpress-fitting the metal substrate and the expanded graphite powder witha mold structurally corresponding to the flow field structure, so as toform a graphite layer covering the surface of the metal substrate fromthe expanded graphite powder.

The present disclosure further provides a bipolar plate for a fuel cell,including a metal substrate having a flow field structure; and aconducting adhesion structure formed on the metal substrate, theconducting adhesion layer including a polymeric adhesive and a pluralityof conductive particles; and a graphite layer formed on the conductingadhesion layer by structurally corresponding to the flow field structureof the metal substrate, the graphite layer being adhered to the metalsubstrate via the conducting adhesion layer, wherein the graphite layeris primarily comprised of expanded graphite powder, and a portion of theexpanded graphite powder is embedded into the conducting adhesion layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are schematic diagrams of a method for producing abipolar plate for a fuel cell according to the present disclosure,wherein FIG. 1E is a photograph of a cross-section of the bipolar platefor a fuel cell;

FIG. 2 is a graph depicting the relationship between the mesh number offlaked graphite and conductivity; and

FIG. 3 is a graph depicting the resistances in the presence or absenceof a graphite layer according to the present disclosure or acommercially available graphite sheet; and

FIGS. 4A and 4B are graphs depicting the voltage decay rates in thepresence or absence of an overlaid graphite layer.

DETAILED DESCRIPTION

In the following, specific embodiments are provided to illustrate thedetailed description of the present disclosure. Those skilled in the artcan easily conceive the other advantages and effects of the presentdisclosure, based on the specification.

Please refer to FIGS. 1A to 1E, which illustrate a method for modifyingthe surface of a metal bipolar plate according to one embodiment of thepresent disclosure.

As shown in FIG. 1A, a metal substrate 10 is prepared. The surface ofthe metal substrate 10 can be, for example, coated by using a scraper,spin-coated, spray-coated, slit-coated or rolling-coated, in advance, toform a conducting adhesion layer 12.

The materials of the above metal substrate, include, but not limited to,at least one of aluminum, copper, nickel, chromium and stainless steel.The material of the metal substrate can also be stainless steel. Themetal substrate can be a flat plate, or one having a flow fieldstructure at the surface thereof. Taking the metal bipolar plate of afuel cell as an example, the metal bipolar plate can be a metalsubstrate having a flow field structure. The flow field structure can bezigzag or snaking, or having a plurality of straight channels. Thethickness of the metal substrate can range from 0.03 mm to 10 mm.

In the embodiment, the conducting adhesion layer includes a polymericadhesive and a plurality of conductive particles. Examples of thepolymeric adhesives are thermosetting resins, photo-curable resins orchemically curable resins. The materials of the conductive particles aremetals, metal alloys, metal carbides, metal nitrides, carbon particlesor a combination thereof. For example, the metals or the metals in themetal carbides or metal nitrides are each at least one independentlyselected from the group consisting of gold, platinum, palladium, nickeland chromium. The metals in the metal alloys are at least two elementsselected from the group consisting of gold, platinum, palladium, nickeland chromium. Usually, the conductive particles made of metals, metalalloys, metal carbides or metal nitrides have particle diameters rangingfrom 10 nm to 100 μm.

In one embodiment, the carbon particles are each at least one selectedfrom the group consisting of a graphite material, a carbon nanocapsule,carbon black, a carbon nanotube and a carbon fiber. The carbon particleshave particle diameters ranging from 10 nm to 100 μm. However, thepowder having excellent conductivity and suitable particle diameters canbe selected based on the practical user needs. Hence, the carbonparticles are not limited to the examples disclosed.

Usually, the thermosetting resins, photo-curable resins or chemicallycurable resins added with the conductive particles are stirred by usinga mechanical equipment or manually, so as to disperse the conductiveparticles in the resins. A conducting adhesion layer is then formed bycoating using a scraper, spin-coating, spray-coating, slit-coating orrolling-coating. The conductive particles take 10% to 70% of the volumeof the conducting adhesion layer.

Then, as shown in FIG. 1B, expanded graphite powder 14 is applied ontothe metal substrate 10 having a conducting adhesion layer 12 at thesurface thereof. The expanded graphite powder 14 is obtained byacidifying and heating flaked graphite. The acids used in theacidification include sulfuric acid, nitric acid or a combinationthereof. Moreover, in one embodiment, the flaked graphite having a meshnumber ranging from 15 to 200 is selected. The flaked graphite comprises40% to 100% of flaked graphite having a mesh number of from 15 to 100,0% to 50% of flaked graphite having a mesh number of from 101 to 200.

In a further embodiment, when the mesh number is smaller than 20, thesurface of the formed graphite layer is rough, and contains many poresand fine crazing. When the graphite layer is used in a fuel cell, theacidic solution is likely to infiltrate and etch the metal substrate.When the mesh number is greater than 200, the formed graphite layer alsohas many voids. Further, the graphite layer has poor strength, such thatit is prone to cracking. In another embodiment, the flaked graphitehaving a mesh number ranging from 30 to 80 is selected.

Please also refer to FIG. 2, which depicts the results of theconductivity measurements using the flaked graphite having a mesh numberranging from 32 to 200. In FIG. 2, the flaked graphite having a meshnumber of 32 has higher conductivity. Although the conductivity of theflaked graphite having a mesh number of 200 decreases, it is stilldesirable. The conductivity of the flaked graphite having a mesh numberof 80 is still higher than 1400 S/cm.

Furthermore, the approach for applying the expanded graphite powder isnot particularly limited. The expanded graphite powder can be appliedonto the conducting adhesion layer mechanically or manually. The amountof the expanded graphite powder used preferably ranges from 5 to 50mg/cm³.

Moreover, please refer to FIG. 3, as compared with the metal substratescomprising a graphite layer according to the present disclosure or acommercially available graphite sheet laid over a conducting adhesionlayer, the metal substrate comprising just the conducting adhesion layerhas a much higher resistance. Further, by comparing the metal substratecomprising the graphite layer and the graphite sheet, it is found thatthe metal structure comprising the graphite sheet has a higherresistance. This results from that the graphite sheet is only looselyattached to the conducting adhesion layer on the top of the metalsubstrate, not like the graphite layer which is embedded into theconducting adhesion layer.

Please refer further to FIGS. 1C and 1D. In FIGS. 1C and 1D, afterapplying the expanded graphite powder 14, press-fitting the metalsubstrate 10 and the expanded graphite powder 14 with a mold 16structurally corresponding to the flow field structure is performed, soas to form a graphite layer 14′ from the expanded graphite powder 14;and the graphite layer 14′ is adhered to the metal substrate 10 via theconducting adhesion layer 12. In one embodiment, the metal substrate isreceived in the mold. For example, the metal substrate can be receivedin the mold after forming the conducting adhesion layer; or the metalsubstrate is placed in the mold after forming the conducting adhesionlayer on the metal substrate. In one embodiment, the metal substrate 10is received in a lower mold 16 a. During press-fitting, an upper mold 16b is used to press against the expanded graphite powder 14.

In another embodiment, only the upper mold can be used to press againstthe expanded graphite powder (not shown in the figures), because themetal substrate has rigidity.

In addition, the metal substrate of the metal bipolar plate has a flowfield structure. When the metal bipolar plate for a fuel cell isapplied, the mold used can structurally correspond to the flow fieldstructure to form a graphite layer. The graphite layer can have acomplete coverage over the substrate by being in compliance with theflow field structure, and thereby forming a dense graphite protectivelayer. Particularly, regardless of what configuration the flow fieldstructure the metal substrate has, the graphite protective layer canindeed be formed by press-fitting with the mold in the horizontal andvertical directions of the flow field structure, as indicated in by thepresent disclosure. Hence, the effect of sufficiently protecting themetal substrate is achieved. Usually, press-fitting the metal substrateand the expanded graphite powder by the mold is conducted at apress-fitting strain ranging from 10 to 1000 kg/cm².

In one embodiment, the thickness of the graphite layer is greater thanthe thickness of the conducting adhesion layer, in order to make thesurface of the graphite layer a uniformly dense surface. For example,the thickness of the graphite layer ranges from 10 μm to 1 mm. Thethickness of the conducting adhesion layer ranges from 0.5 to 500 μm.

Furthermore, as shown in FIG. 1E, the conducting adhesion layer 12 is apolymer. Hence, when the expanded graphite powder 14 is embedded in thepolymer during press-fitting, the impedance at the hetero-junction iseliminated, the etching of the metal substrate 10 by the acidic solutionis avoided, and the bonding strength is increased.

According to the aforesaid method, the present disclosure furtherprovides a bipolar plate for the fuel cell, including: a metal substratehaving a flow field structure; and a conducting adhesion layer formed onthe metal substrate, the conducting adhesion layer having a polymericadhesive and a plurality of conductive particles; and a graphite layerformed on the conducting adhesion layer by structurally corresponding tothe flow field structure of the metal substrate, the graphite layerbeing adhered to the metal substrate via the conducting adhesion layer,wherein the graphite layer is primarily comprised of expanded graphitepowder, and a portion of the expanded graphite layer is embedded intothe adhesive, wherein the expanded graphite powder is obtained byacidifying and heating flaked graphite having a mesh number of from 15to 200.

In one embodiment, the thickness of the graphite layer is greater thanthe thickness of the conducting adhesion layer. For example, thethickness of the graphite layer ranges from 10 μm to 1 mm, and thethickness of the conducting adhesion layer ranges from 0.5 to 500 μm.

The materials of the conductive particles are metals, metal alloys,metal carbides, metal nitrides, carbon particles or a combinationthereof. For example, the metals or the metals in the metal carbides ormetal nitrides are each at least one independently selected from thegroup consisting of gold, platinum, palladium, nickel and chromium. Themetals in the metal alloys are at least two elements selected from thegroup consisting of gold, platinum, palladium, nickel and chromium.Usually, the conductive particles take of metals, metal alloys, metalcarbides or metal nitrides have particle diameters ranging from 10 nm to100 μm.

In one embodiment, the carbon particles are each at least one selectedfrom the group consisting of a graphite material, a carbon nanocapsule,carbon black, a carbon nanotube and a carbon fiber. The carbon particleshave particle diameters ranging from 10 nm to 100 μm. Further, theconductive particles take 20% to 80% of the volume of the conductingadhesion layer.

Test Example

In the test example, 1 mm stainless steel 316 L was selected as amaterial of the metal substrate with a flow field structure. The epoxyresin and carbon black were mixed to form a composition of theconducting adhesion layer coated on the metal substrate, and the ratioof carbon black was 40 volume %. The other conductive particles such ascarbon, metal nitride and metal carbide, could also be added to theconductive resin. Then, expanded graphite, which was obtained byacidifying and heating flaked graphite, was disposed over the substratewith a conducting adhesion layer with a thickness of 50 μm. The mixedflaked graphite including 75% of the flaked graphite having a meshnumber of 50, and including 25% of the flaked graphite having a meshnumber of 150. The amount of the expanded graphite was 25 mg/cm².Press-fitting with the mold in the horizontal and vertical directions ofthe flow field structure was used to form the graphite layer, and thestrain of the press-fitting was 500 kg/cm². The thickness of graphitelayer was 150 μm. Therefore, a metal bipolar plate with a conductingadhesion layer and a graphite layer was obtained by the process.

Please refer to FIGS. 4A to 4B, which depict the voltage decay rates ofthe bipolar plates with and without an overlaid graphite layer.

The fuel cell testing instrument used in the test example tookmeasurements according to the constant-current discharge testing method.As shown in FIG. 4A, when only the 316 L stainless steel is used as thebipolar plate, the voltage decay rate is already greater than 60 μV/h atthe 1000^(th) hour.

As shown in FIG. 4B, the conducting adhesion layer has a thickness of 40μm. The conducting adhesion layer further includes 60% ofcarbon-containing conductive particles. The conducting adhesion layer isoverlaid with the graphite layer, which is made of expanded graphite.The expanded graphite is obtained by acidifying and heating flakedgraphite having a mesh number of 30, and press-fitting the flakedgraphite at a press-fitting strain of 500 kg/cm². The bipolar plate hasa voltage decay rate of still less than 10 μV/h, after being tested for1000 hours.

The foregoing descriptions of the detailed embodiments are onlyillustrated to disclose the principles and functions of the presentdisclosure, and not used to limit the scope of the present disclosure.It should be understood by those in the art that alterations can be madeto the above embodiments, without departing from the spirit and scope ofthe disclosure of the present disclosure. The scope of the disclosure ofthe present disclosure shall fall within the scope of the appendedclaims.

1. A bipolar plate for a fuel cell, comprising: a metal substrate havinga flow field structure; a conducting adhesion layer formed on the metalsubstrate, and comprising a polymeric adhesive and a plurality ofconductive particles; and a pure graphite layer formed on the conductingadhesion layer and structurally corresponding to the flow fieldstructure of the metal substrate, the graphite layer being adhered tothe metal substrate via the conducting adhesion layer, wherein thegraphite layer comprises expanded graphite powder, and a portion of theexpanded graphite powder is embedded into the conducting adhesion layer.2. The bipolar plate of claim 1, wherein the expanded graphite powder isobtained by acidifying and heating flaked graphite having a mesh numberranging from 20 to
 200. 3. The method of claim 1, wherein the flakedgraphite comprises 40% to 100% of flaked graphite having a mesh numberof from 15 to 100, 0% to 50% of flaked graphite having a mesh number offrom 101 to
 200. 4. The bipolar plate of claim 1, wherein the graphitelayer has a thickness ranging from 10 μm to 1 mm.
 5. The bipolar plateof claim 1, wherein the portion of the expanded graphite powder isembedded into the conduction adhesion layer ranging from 5% to 50%compared with the conducting adhesion layer.
 6. The bipolar plate ofclaim 1, wherein the conducting adhesion layer has a thickness rangingfrom 0.5 to 500 μm.
 7. The bipolar plate of claim 1, wherein theconductive particles are made of a material selected from the groupconsisting of a metal, a metal alloy, a metal carbide, a metal nitride,a carbon particle and a combination thereof.
 8. The bipolar plate ofclaim 7 wherein at least one of the metal, the metal carbide and metalnitride comprises gold, platinum, palladium, nickel or chromium.
 9. Thebipolar plate of claim 7, wherein the metal alloy comprises at least twoelements selected from the group consisting of gold, platinum,palladium, nickel and chromium.
 10. The bipolar plate of claim 7,wherein the conductive particles have particle diameters ranging from 10nm to 100 μm.
 11. The bipolar plate of claim 7, wherein the carbonparticle is made of at least one selected from the group consisting of agraphite material, a carbon nanocapsule carbon black, a carbon nanotubeand a carbon fiber.
 12. The bipolar plate of claim 7, wherein theconductive particles take 10% to 70% of a volume of the conductingadhesion layer.
 13. The bipolar plate of claim 7, wherein the polymericadhesive is at least one selected from the group consisting of athermosetting resin, a photo-curable resin and a chemically curableresin.
 14. The bipolar plate of claim 1, wherein the metal substrate hasa thickness ranging from 0.03 mm to 10 mm.